docs(runtime): fix host-tag comment drift after tui- prefix rename

Adversarial review found stale bare-host-tag references the per-file sed couldn't
reach (they live in comments/docs). Code was clean — no contamination, no public
API leakage, root/text-leaf/comment asymmetry consistent. Updated:
- vite.config.ts isCustomElement comment (<box>/<text> -> <tui-box>/<tui-text>)
- component-authoring.md split-table Text row (virtual-text/text -> tui-*)
- box.vue / useBoxMetrics.ts / use-box-metrics.test.tsx "the real `box` host node"
  comments -> `tui-box`

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/runtime/vite.config.ts

@@ -13,7 +13,7 @@ export default defineConfig({
   // `VueVite` parses `.vue` SFCs in the TEST/dev graph (unit tests may import the
   // .vue components directly, e.g. host/text-measure.test.ts). The `pack` build has
   // its own `Vue` rolldown plugin below; both need `isCustomElement` so the host
-  // tags (`<box>` / `<text>` / …) inside SFC templates compile to raw element
+  // tags (`<tui-box>` / `<tui-text>` / …) inside SFC templates compile to raw element
   // vnodes instead of being resolved as components.
   plugins: [
     vueJsx(),